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EDIT: While following up on the two suggestions I determined that the following would work for writing the Strings to a file as Byte arrays. But now I need to figure out how to read the Byte arrays back into Strings. I could use help with that please... Is there a FileInputStream component that is the reverse equivalent of this FileOutputStream? This works for writing the Strings as Byte arrays.

try {
    FileOutputStream fos = openFileOutput("stack.txt", Context.MODE_PRIVATE);
    for (String item : stack)
       fos.write(stack.pop().getBytes());
    fos.close();
    sb.append("The Stack is saved.\n");
    } catch (IOException e) {
         sb.append(e.toString());
    }
displayText(sb.toString());

I am trying to write Strings from an ArrayDeque to a file. I am using the ArrayDeque as a Stack. The strings are of varying length. I get this error on opening the file for output...

java.io.FileNotFoundException: stack.txt: open failed: EROFS (Read-only file system)

I think I have seen all of the posts on this topic but with no help. My two code snippets are:

  1. In onCreate()...
root = new File(getExternalFilesDir(null), "/HTMLSpyII/");
if (!root.exists()) {
    if (!root.mkdirs()) {
        runTimeAlert("File path does not exist or\n" +
            "Unable to create path \n" + root.toString());
            }
        }
  1. In the program...
try {
    DataOutputStream dos = new DataOutputStream(
        new FileOutputStream("stack.txt")); // program fails here
    for (String item : stack)
        dos.writeUTF(stack.pop());
        dos.close();
        sb.append("The Stack is saved.\n");
    } catch (IOException e) {
        sb.append(e.toString());
    }
    displayText(sb.toString());

I have parallel code for reading them back using DataInputStream with FileInputStream. I am missing something. Is it perhaps some initial preparation before creating the file?

Doing some research I have a feeling that the problem may be that I am not yet familiar with the requirements for using the new app-specific, persistent, internal/external storage? But I do not know so I am looking for some guidance, please :)

Mick
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  • Does this answer your question? [Trying to create a file in Android: open failed: EROFS (Read-only file system)](https://stackoverflow.com/questions/15711098/trying-to-create-a-file-in-android-open-failed-erofs-read-only-file-system) – Joachim Sauer May 16 '21 at 16:42
  • `new FileOutputStream("stack.txt"));` Replace by `File file = new File(root, "stack.txt"); new FileOutputStream(file);` – blackapps May 16 '21 at 17:02
  • @Joachim Sauer I did not find an answer there. Interesting stuff but not a File technique which I might use to open for write and read back a series of strings :( blackapps I was unable to figure out how to make the replacement work that you suggested :( I need to use readUTF and writeUTF which need ObjectInputStream and ObjectOutputStream. I edited the post with my new approach. It seems that writeUTF/readUTF is the only pair that write Strings to a file and then read them back. They are part of java.io.ObjectOutputStream. I need to find a way to open/close the file without crashing. – Mick May 16 '21 at 23:05

1 Answers1

1

This is the outline of a complete solution to the problem. Hope this helps someone, sometime...

  1. The problem is: To use an ArrayDeque as a Stack for saving Strings, and use a file for Save and Restore of the stack.

  2. The key parts of the solution are: to use ObjectOutputStream and ObjectInputStream for saving and restoring the stack to external storage. to use getExternalFilesDir() to open the file for save and for restore.

  3. To open the file for Saving the current stack contents...

File stackRoot = new File(getExternalFilesDir(null), "/AppName/stack.dat");
    FileOutputStream fos = new FileOutputStream(stackRoot);
    ObjectOutputStream oos = new ObjectOutputStream(fos);
  1. To empty the Stack into the file...
for (String item : stack) oos.writeObject(item);
oos.close();
  1. To open the file for Restoring the saved stack contents...
File stackRoot = new File(getExternalFilesDir(null), "/AppName/stack.dat");
    FileInputStream fis = new FileInputStream(stackRoot);
    ObjectInputStream ois = new ObjectInputStream(fis);
  1. To empty the file into the Stack...
while (true) stack.addLast((String) ois.readObject());
Mick
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